This invention relates to improvements in electric motors of the brushless type of the kind for use particularly although not exclusively in power assisted steering systems for vehicles in which the motor is adapted to apply an assistance torque to a portion of steering shaft in response to a motor drive signal. The invention also relates to electric power assisted steering systems of the kind set forth.
In one known power assisted steering system, a first portion of the shaft is operatively connected to a steering wheel, a second portion of the shaft is operatively connected to at least one steerable wheel, a torque sensor is adapted to produce an output signal indicative of torque applied to the first portion of the shaft by a driver of the vehicle, and control means are adapted to process the output signal in order to produce the motor drive signal.
The first and second portions of shaft may comprise respective portions of a single shaft , or may each comprise a separate shaft.
The purpose of an electric power assisted steering system of the kind set forth is for the motor to apply an assistance torque to the second portion of shaft which makes it easier for the driver to turn the steering wheel. Typically, the motor is controlled using a pulse width modulation strategy with the motor drive signal being proportional to the torque required from the motor.
In the prior art, such as our earlier application GB 9619175 the motor comprises a direct current brushless motor driven by an electrical circuit in which the motor phase windings are connected in a bridge. Each phase is connected into a separate arm of the bridge between a positive and negative (or ground) supply line. A respective top switching device is connected in series in each arm between a phase winding and the positive supply, and a respective bottom switching device is connected in each arm between a phase winding and the negative (or ground) supply line. Thus, for a 3-phase motor the bridge has 3 arms, each comprising a top and a bottom switching device. The motor can be controlled by pulse width modulating the top and bottom switching devices in the arms in any known manner to cause the motor to rotate.
A problem with brushless permanent magnet motors is that a failure mode exists in which the motor phase windings may be shorted together to form a loop around at least two phases. This typically occurs if two top and/or two bottom devices stick or otherwise fail in a closed state to form a closed path around the electrical circuit.
In such a failure mode, the motor will act as a generator, and any attempt to rotate the rotor within its phase windings will induce a braking force which opposes the movement. In a steering system, this is highly undesirable as in a worst case the braking force may be sufficiently high as to prevent manual rotation of the second portion of shaft by the driver.
The electric motor disclosed in GB 9619175 is shown in FIG. 6 of the accompanying drawings. It includes a rotor 100 carrying a plurality of surface mounted permanent magnets of arcuate section. The magnets 101 are held in position by a non-magnetic sleeve (not shown) of approximately 0.3 mm to 0.5 mm which fits within the air gap 102 between the rotor magnets and the stator 200. Flux passes through this air gap and non-magnetic sleeve to complete a magnetic circuit between poles on the stator and the rotor magnets.
Because of the presence of the non-magnetic sleeve, the gap between stator and rotor is relatively large which results in a low motor reactance with in turn means high levels of braking torque.
It has been proposed to overcome this problem in several ways. Firstly, it is known to provide a clutch between the motor and the shaft. This is disengaged in the event of a motor failure so that the motor will not resist attempts to turn the output shaft manually.
In an alternative, it is known to connect the motor phase windings in a star formation and provide one or more isolating switches at the star point. In the event of a failure, the isolating switches are opened so that it is impossible for a closed loop to form. However, it is still possible for a failure of the isolating switches to occur which would allow a closed loop to form.
An object of the invention is to ameliorate the problems of braking force in a motor in an electric power assisted steering system.
In accordance with a first aspect, the invention provides an electric motor of the brushless type so constructed and arranged as to produce a low braking torque in the event of a failure.
By low braking torque we mean that the braking torque is less than that produced using a standard surface mounted magnet motor. For example, the peak braking torque seen in the steering column due to peak motor braking torque may be less than 70 Nm, or perhaps less than 30 Nm or less than 20 Nm. This low torque ensures that, although the braking torque will be felt through the steering wheel it can be easily overcome by the driver. The low braking torque may be achieved by providing a motor with a high reactance. For example, the impedance of the motor may be greater than 100 mHenry, or preferably greater than at least 130 mHenrys.
The motor may be adapted to provide a peak torque during motoring to the steering shaft of at least 50 Nm, or preferably at least 100 Nm or more.
The motor may, in one especially advantageous arrangement comprise an interior permanent magnet motor having a plurality of permanent magnets buried within a rotor structure.
By xe2x80x9cburiedxe2x80x9d we mean that the magnets are either flush with or below the level of the circumference of the rotor structure. When below the level of the circumference, the magnets may be covered by a magnetic or magnetically susceptible cover to provide a continuous smooth surface to the rotor.
Preferably, the magnets are buried wholly within the rotor structure so that the rotor provides a substantially continuous outer circumferential surface. The magnets may therefore be totally enclosed within the rotor structure.
By providing a buried magnet motor the non-magnetic retaining sleeve required for surface mounted magnets motors can be eliminated, reducing the size of the air gap between the stator and rotor. This results in a motor having a higher impedance and hence lowered braking torque when a failure occurs.
An advantage of an interior permanent magnet motor over a conventional surface mount motor is that there is no longer a need to provide a retaining sleeve to hold the magnets in place. This enables the magnets to be positioned closer to the stator, maximising the value of the motor reactance which decreases the braking torque peak value.
Preferably, each magnet is received within a slot in the rotor below the rotor surface. The magnets may comprise rectilinear flat bar magnets having planar upper and lower faces which are received within slots in a rotor. The rotor may have a unitary construction. Flat magnets are cheaper than arcuate magnets, and are easier to produce.
This arrangement eliminates the need for the retaining sleeve, effectively placing the edges of the magnets closer to the stator and hence reducing the space therebetween.
Preferably, the rotor is of steel or other suitable metallic material. This provides an improved magnetic flux linkage between the stator poles and the rotor magnets.
Of course, other typos of motor could be employed which provide a low braking torque and thus fall within the scope of the present invention.
The rotor may be hollow whereby an axle can be passed through the rotor to attach the rotor to an output shaft. Alternatively, the output shaft could form an integral part of the rotor. Preferably the gap between the stator poles and the outer surface of the rotor is less than 0.5 mm, or ideally less than 0.3 mm. To minimise the braking torque, this should be kept as small as possible within design tolerances. To this end, the slots are preferably provided as close to the circumference of the rotor as is mechanically feasible to place the magnets as close as possible to the stator poles.
The motor may comprise at least two or more phases, although three phases is preferred.
The motor phases may each comprise a number of phase windings connected in series or in parallel. Each phase may be connected either in a Delta or a Wye formation.
In accordance with a second aspect, the invention provides an electric power assisted steering system of the kind set forth in which the motor comprises an interior permanent magnet motor having a plurality of permanent magnets buried within a rotor structure and a number of phase windings provided on a stator arranged concentrically around the rotor structure.
In accordance with a third aspect, the invention comprises a mechanical system comprising an output shaft driven by a motor, in which the motor comprises an interior permanent magnet motor comprising a rotor having a plurality of magnets buried at least partially below the surface of the rotor.
The system may comprise in one arrangement, an electric power assisted steering system whereby the output shaft is operatively connected to a steering shaft. Alternatively, if may comprise any other direct drive mechanical system in which it is desirable to minimise braking torque during motor failure.